U.S. Pat. No. 7,463,398 B2, 2008, by Feenstra et al., describes an electrowetting display where the spread of two immiscible liquids: hydrophilic and hydrophobic, over a hydrophobic surface is controlled by an external voltage source. At non-energized state, the non-polar hydrophobic fluid occupies and spreads over most of the hydrophobic area. When an electrical field is applied between the polar hydrophilic fluid and the electrode underneath the hydrophobic area, the hydrophobic surface becomes more hydrophilic, increasing the wetting angle of the non-polar fluid and decreasing the wetting angle of the polar fluid. At a sufficiently high voltage, the non-polar fluid eventually contracts and is displaced by the polar fluid. Removing the electrical field will return the fluids to its initial wetting state. The spread and contraction of the non-polar fluid can be used to induce optical contrast of the electrowetting device by applying color to that fluid. One disadvantage of this type of display is the rigid nature of the structure, namely the silicon substrate on which all of the display components are laid. As a result, this type of display cannot be used for applications that need a certain degree of flexibility, such as a wrapable display. In addition, the use of water as the polar liquid limits the operating temperature from −20 to 60° C. (Heikenfeld, 2011 p 142). Furthermore, electrowetting displays are slow for large display application that requires tens of millimeter pixel sizes such as billboards, because the response time is proportional to the square of the pixel size. Another disadvantage is the presence of transparent electrode positioned between the color active component and the observer, which decreases the contrast ratio and/or the color saturation of the device.
US Published Patent Application 2010/0208328 A1, 2010, by Heikenfeld et al., describes an electrofluidic device containing polar and non-polar fluids which are driven by electrowetting. Contrary to the electrowetting device of Feenstra described above, complete displacement of non-polar fluid by the polar fluid in this device is accomplished within a viewable confined fluid chamber. According to the author, this design enables higher contrast ratio and provides more control to the gray-scale switching compared to the Feenstra design. Since the underlying physics and device construction are similar to an electrowetting display, these displays, however, share the same disadvantages mentioned above.
U.S. Pat. No. 7,619,609 B2, 2009, by Fork, describes an array of display pixels that uses rows and columns of fluid manifolds connected to centralized pumps that drive fluids into a pixel container. The amount of fluids supplied into the pixels is regulated by a series of valves controlled by electrical current. During fluid withdrawal, pressured air was delivered into the top of pixel container to push the fluid out of the pixel container. Manufacturing such a pixel display array with its multiple components (fluids, electrical signals and pressurized air) within a single pixel, will prove challenging.